What is the correct way to cast when using ATL and IUnknownPtr? - com

Question

During the modification of an existing ATL COM object I came across an article from the "The Old New Thing" blog called "The ways people mess up IUnknown::QueryInterface" and there was a discussion in the comments section that started when one of the respondents (Norman Diamond) pointed out that that in one of the article's examples that the cast to void** was incorrect.
However when I try and correct my code to do the casting properly I end up with a memory leak.
The example was as follows:
IShellFolder *psf = some object;
IUnknown *punk = NULL;
psf->QueryInterface(IID_IUnknown, (void**)&punk);
Norman said
punk is not a void*. punk is an IUnknown*.
void** is not a universal pointer type. void* is a universal pointer type, and char* and relatives are grandparented in to be equivalent in that way, but void** is not.
If you want to obey the calling convention and avoid horrible deaths, you have to do this:
IUnknown *punk;
void *punkvoid;
psf->QueryInterface(IID_IUnknown, &punkvoid);
punk = (IUnknown *)punkvoid;
Lots of other MSDN contributors made the same identical mistake.... some people might say that it works in all VC++ implementations to date, but that doesn't make it correct code, and it's still violating the calling convention.
In light of this I went to change my old code - which was as follows:
#include <comdef.h>
...
HRESULT FinalConstruct()
{
if (m_dwROTCookie != 0)
return E_FAIL;
//Check whether there already is an instance of the Object
IUnknownPtr pUnk = NULL;
if (GetActiveObject(CLSID_Object, NULL, &pUnk) == S_OK)
{
TRACE_WARNING("An instance of Object already exists in the current context");
return S_OK;
}
HRESULT hr = QueryInterface(IID_IUnknown, reinterpret_cast<void **>(&pUnk));
hr = RegisterActiveObject(pUnk, CLSID_Object, ACTIVEOBJECT_WEAK, m_dwROTCookie);
if (FAILED(hr))
return hr;
hr = CoLockObjectExternal(pUnk, TRUE, TRUE);
pUnk = NULL;
ATLASSERT(m_dwRef == 2);
return hr;
}
I then changed it as follows:
HRESULT FinalConstruct()
{
if (m_dwROTCookie != 0)
return E_FAIL;
//Check whether there already is an instance of the Object
IUnknownPtr pUnk = NULL;
if (GetActiveObject(CLSID_Object, NULL, &pUnk) == S_OK)
{
TRACE_WARNING("An instance of Object already exists in the current context");
return S_OK;
}
void* pUnkVoid = NULL;
HRESULT hr = QueryInterface(IID_IUnknown, &pUnkVoid);
if (SUCCEEDED(hr)
{
pUnk = reinterpret_cast<IUnknown*>(pUnkVoid);
hr = RegisterActiveObject(pUnk, CLSID_Object, ACTIVEOBJECT_WEAK, m_dwROTCookie);
if (FAILED(hr))
return hr;
hr = CoLockObjectExternal(pUnk, TRUE, TRUE);
pUnk = NULL;
}
ATLASSERT(m_dwRef == 2);
return hr;
However now my application has a memory leak from this the COM Object

Answer 1

You likely have a memory leak because you call GetActiveObject() and QueryInterface() which upon success increment the reference count on the object, but don't call Release() later to decrement the reference count.

Answer 2

Mmm, I think that rather than assigning the void* to pUnk I should be using:
pUnk.Attach(reinterpret_cast<IUnknown*>(pUnkVoid));